Concrete Without Steel?

Q.Does the addition of fibers
eliminate the need for steel in a concrete-slab
basement floor?

A.Bill Palmer, former
editor of Concrete Construction magazine and
president of Complete Construction Consultants in
Lyons, Colo., responds: For the last 30 years
or so, synthetic fibers made from nylon,
polypropylene, and other plastics have been used in
low dosage rates of around 1.5 to 3.0 pounds per
cubic yard to help reduce plastic shrinkage cracks
(the ones that occur immediately following
placement) in concrete. Plastic fibers can also
increase a slab’s impact resistance.

However, they’re ineffective at
controlling crack width and location;
that’s the main purpose of wire mesh,
welded wire fabric (WWF), and regular reinforcing
steel. But even these metal reinforcements
aren’t very effective if they’re
not chaired up properly and end up in the bottom of
the slab rather than in the upper half.

Research into polymer fibers and improvements in
concrete admixtures have recently led to the
development of concrete mixed with much higher
dosages of “structural” fibers.
Sometimes called macro polymeric fibers, these are
much longer (about 2 1/4 inches in length) and
thicker than regular fiber additives, and are added
at dosage rates of as high as 30 pounds per cubic
yard. (A more typical rate is 7 1/2 pounds per
cubic yard.) The resulting “high-volume
synthetic fiber” concrete has yet to gain
widespread acceptance — perhaps because of
concerns about workability — but studies
show that it resists cracking better than regular
steel-reinforced concrete. So it’s
possible that structural fibers may eventually
replace WWF and other traditional crack-control
reinforcements.